Organic electroluminescence (hereinafter sometimes abbreviated to “EL”) displays have been developed enthusiastically since organic EL elements of the multilayer type produced by a vacuum deposition method were marketed by Kodak Company, and are presently coming to be put to practical use.
Such a multilayer type organic electroluminescent element is configured by superposing a plurality of organic layers (an emissive layer, hole injection layer, hole transport layer, electron transport layer, etc.) between an anode and a cathode. Since the organic electroluminescent element itself emits light, the display has a wide viewing angle and attains high visibility. This display is also attracting attention from the standpoint of space saving, etc. because the organic EL element is of a thin film type.
Unlike the light-emitting diodes and cold-cathode tubes which have been in practical use, organic electroluminescent element have a marked feature that the elements emit light areally. Among the applications where this feature is utilized are lightings and the backlights of full-color displays in which color filters are used.
In the case where organic electroluminescent element are used in a lighting, it is possible to obtain a desired color of luminescence by causing elements of multiple colors to emit light and mixing the colors. For example, in the case where red, green, and blue luminescent materials are used, white luminescence is possible. In the case where red and green luminescent materials are used, yellow luminescence is possible.
One method for mixing colors of luminescence is to incorporate luminescent materials of multiple colors into the same emissive layer. Examples of methods for forming an emissive layer include a vacuum deposition method and a wet film formation method.
In the case of the vacuum deposition method, at least four compounds, including red, green, and blue luminescent materials and a charge transport material, must be simultaneously vacuum-deposited (see, for example, patent document 1).
Meanwhile, in the case of the wet film forming method, luminescent materials and a charge transport material are used in the form of a solution in solvent, and there are cases where these materials are low-molecular compounds or where a high-molecular compound is used as the charge transport material (see, for example, patent document 2).